"It's no secret that Mozilla has been working on a mobile OS. Previously codenamed Boot2Gecko, the project focused on a purely HTML5 based system that worked in many ways like current mobile devices. As the project grew into Mozilla OS, the company has laid out a partnership with ZTE that will have real world devices in certain markets early next year. Testing for this OS had previously consisted of a compiled ROM that would be flashed over a handful of Android devices. Now, Mozilla has moved into full fledged product evaluation mode with their own custom developer phone." Looks decent - and a better solution for testers and developers than custom ROMs.

Applications are developed in HTML5. Apple tried this years ago and it didn't work. Developers have shown again and again to prefer native.
I'm sorry Mozilla, your dream of a web based OS where apps are HTML5 is too 2007.